I would first say that the difference between 114 degrees (7mm) and 100 degrees (9mm) is very large in U/W photography, I have used both lenses for some time.

Second if you are adding a color filter to the 7-14 zoom it would stand to reason that for the same results you would need to add it to the 9-18mm zoom and you would see no difference between the two using them the same way.

BEWARE, I AM NOT RECOMMENDING THIS but in the film days we added the filter to the rear of the lens by cuting the filter the size of a dime. This is the hard part, put grease or rubber glue on the metal around the rear lens element then place the filter in place over the lens with twezzers and push to hold in place. If the filter falls off it could cause large problems, if you get glue or grease on the lens element big problem.

I have also used the 12-35 above and below water and it is a great lens. Be aware that rumors of a new (some time this year) like Olympus lens with even better image quality have been flooting around the net.

My point with regard to the color filter on the 7-14 vs the 9-18 was that if you have to rig it up on the 7-14 then you stand a good chance of smudging something or not lining it up straight. Where as on the 9-18 it's a simple screw on filter so less chance for error in application.

Yes, filter will have same effect on each lens, assuming it's installed without any problems.

But, if you absolutely need the 7mm view, then that's what you go with.